1. Field of the Invention
The invention relates to a combustion chamber, for example for a gas turbine, with at least one premixing burner which is arranged in a dome communicating with a plenum and which is fastened on the outlet side to a front plate limiting the combustion space of the combustion chamber.
In such a premixing burner the combustion air is supplied from the dome,
and fuel is injected via nozzles intensively intermixed with the combustion air within a premixing space prior to ignition.
2. Discussion of Background
Combustion with the highest possible excess air number (which is defined as the ratio of the actual air/fuel ratio to the stoichiometric air/fuel ratio), is generally determined, on the one hand, by the fact that the flame still burns at all and, further, by the fact that too much CO does not occur. Combustion of this type reduces not only the quantity of harmful NO.sub.x, but, moreover, also ensures that other harmful substances are kept low, in particular, as already mentioned, CO and unburnt hydrocarbons. This makes it possible to select a higher excess air. In addition, although larger quantities of CO occur initially, these can nevertheless react further to form CO.sub.2, so that, finally, the CO emissions remain low. On the other hand, however, only little additional NO.sub.x forms on account of the high excess air. Since a plurality of tubular elements accomplish the premixing in this known combustion chamber, during load regulation only as many elements are in each case operated with sufficient fuel to ensure that the optimum excess air number is obtained for the particular operating phase (starting, part load, full load).
The so-called premixing burners of the double-cone design can be designated as flame-holding burners of the type mentioned at the outset. Such double-cone burners are known, for example, from U.S. Pat. No. 4,932,861 to Keller et al and will be described later with reference to FIGS. 1 and 2. The fuel, gas there, is injected in the inlet gaps into the combustion air flowing forwards from the compressor, by way of a row of injector nozzles. These are usually distributed uniformly over the entire gap.
In order to achieve a reliable ignition of the mixture in the downstream combustion chamber and a sufficient burn-up, an intimate mixing of the fuel with the air is necessary. Good intermixing also contributes to avoiding so-called "hot spots" in the combustion chamber, which lead inter alia, to the formation of the undesirable NO.sub.x.
However, all combustion chambers with premixing burners have a shortcoming that the limit of flame stability is nearly reached, at least in the operating states in which only some of the burners are operated with fuel, or in which the individual burners are loaded with a reduced quantity of fuel. In fact, on account of the very lean mixture and the low flame temperature resulting from this, under typical gas-turbine conditions the extinguishing limit is already reached when the excess air number is approximately 2.0.
This fact leads to a relatively complicated mode of operation of the combustion chamber with a regulation which involves a correspondingly high outlay. Another possibility for widening the operating range of premixing burners is seen in assisting the burner by means of a small diffusion flame. The fuel which this pilot flame receives is pure or at least inadequately premixed, thus on the one hand leading admittedly to a stable flame, but on the other hand resulting in the high NO.sub.x emissions typical of diffusion combustion.